The study of liver development and regeneration has broad therapeutic implications for human liver diseases. Enhanced regeneration could improve liver transplantation outcomes for both donors and recipients, while identifying factors required for liver development may one day lead to the creation of a functioning liver ex vivo [1]. The zebrafish liver is remarkably similar to that of its mammalian counterparts, in structure, function and development [2]. This fact, combined with the optical clarity of embryos, large sample size and genetic tractability, makes zebrafish an exceptional model to study hepatogenesis. Liver development in zebrafish and mammals is loosely divided into three overlapping stages: 1) hepatoblast specification, 2) budding and differentiation and 3) outgrowth (i.e. expansion of liver size) and morphogenesis (i.e. attainment of characteristic 3-dimensional shape). While the first two stages of liver development are relatively well-characterized, only a few genes are known to be important for hepatic outgrowth, and liver morphogenesis remains a virtual mystery. UHRF1, an important cell cycle regulator, epigenetic modulator and potential chemotherapeutic target [3, 4], is expressed in the developing liver and was found to be essential for hepatic outgrowth and morphogenesis. Zebrafish mutants with a loss of function mutation in the uhrf1 gene display small, ball-shaped livers among other developmental abnormalities and do not survive past 6 days post fertilization (dpf). In addition, adult uhrf1 heterozygotes fail to regenerate their livers after partial hepatectomy, a defect found to be specific to the liver [5]. Thus, uhrf1 is required for hepatocyte proliferation during liver development and liver regeneration. However, it is not clear whether the failure of hepatocyte proliferation during the above processes is due to the loss of Uhrf1 within hepatocytes. The hepatocyte-autonomous and non-hepatocyte-autonomous contributions of UHRF1 to liver size (counting hepatocyte number, measuring liver volume, assessing hepatocyte proliferation (PCNA)) and shape (measuring circularity of liver lobes, visualization hepatic architecture through markers of non-parenchymal cells) will be assessed in the context of liver development and liver regeneration following partial hepatectomy. For these purposes, 1) uhrf1 mutants and heterozygotes in which UHRF1 is replaced in hepatocytes and 2) uhrf1 mutants that have received transplanted wildtype hepatic progenitor cells will be used. These findings will not only illustrate the fundamental properties of liver development and regeneration but will also highlight the mechanism by which UHRF1 functions to control cell proliferation. PUBLIC HEALTH RELEVANCE: Stimulating one's own cells to develop and regenerate while avoiding carcinogenesis may represent a viable strategy for patients suffering from end-stage liver disease. This proposal will define the role of UHRF1, an important cell cycle regulator, epigenetic modulator and possible oncogene, in governing liver development and regeneration;the results will also have implications for the design of effective cancer therapeutics.